1. Field of the Invention
The present invention relates to a wireless medium access control (MAC) protocol, and more particularly relates to a hybrid wireless MAC protocol which uses a combination of slot (bandwidth) allocation, a variation on conventional token passing and polling for use in regulating isochronous traffic transmission.
2. Description of Related Art
The wireless communication market has lately enjoyed tremendous growth and is now capable of reaching every place on earth. Hundreds of millions of people exchange information every day using pagers, cellular telephones and other wireless communication products. Wireless communication has broken the harnesses of wireline networks, allowing users to access and share information on a global scale nearly everywhere they venture.
Standard LAN protocols (wireline), such as ETHERNET(trademark), operate on wireline networks using various MAC protocols, e.g., carrier sense multiple access with collision detection (CSMA/CD), at fairly high speeds with inexpensive connection hardware which provides an ability to bring digital networking to practically any computer. Until recently, however, LANs were limited to physical, hard-wired (wireline) infrastructure. Even with phone dial-ups, network nodes were limited to access through wireline connections. Wireline communications, however, have set the stage for wireless communications.
Since the recent development of wireless LANs, many network users, such as mobile users in business, the medical professions, industry, universities, etc., have benefited from the enhanced communication ability of wireless LANs, i.e., increased mobility. Uses for wireless network access are practically unlimited. In addition to increased mobility, wireless LANs offer increased flexibility. Compared to wireline counterparts, however, wireless networks are known to have much less bandwidth, and hence it is highly desirable to utilize the wireless link bandwidth efficiently.
The IEEE standard for wireless LAN protocol is identified as xe2x80x9cStandard for Information Technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific Requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY),xe2x80x9d 1999, which will be referred to hereinafter as IEEE 802.11. IEEE 802.11 specifies parameters of both the physical (PHY) and medium access control (MAC) layers of the network. The PHY network handles transmission of data between nodes by either direct sequence spread spectrum, frequency-hopping spread spectrum, or infrared (IR) pulse position modulation. IEEE 802.11 makes provisions for data rates of either 1 Mbps or 2 Mbps, and calls for operation in the 4.4-2.4835 GHz band (spread spectrum), and 300-428,000 GHz for IR transmission.
In accordance with IEEE 802.11, there are two different ways to configure a network: ad-hoc and infrastructure. An example of an ad-hoc wireless network would include employees in various places using small computers and wireless links to implement an xe2x80x9cad-hocxe2x80x9d wireless network to facilitate the business meeting xe2x80x9con the flyxe2x80x9d. Such ad-hoc networks may be brought up and torn down in a very short time as needed, either around the conference table and/or around the world. In order to maintain such networks, algorithms such as the spokesman election algorithm (SEA) have been designed which xe2x80x9celectxe2x80x9d one machine as the base station (master) of the ad-hoc wireless network, the remaining network members as slaves.
An example of an xe2x80x9cinfrastructurexe2x80x9d wireless network under IEEE 802.11 uses fixed network access points with which mobile nodes can communicate. These network access points are sometimes connected to landlines to widen the LAN""s capability by bridging wireless nodes to other wired nodes. If service areas overlap, handoffs can occur (which operation is very similar to conventional cellular technology). Regardless of the wireless LAN configuration, however, the WLAN still requires a medium access control protocol.
The MAC layer is a set of protocols which is responsible for maintaining order in the use of a shared medium. IEEE 802.11 specifies a carrier sense multiple access with collision avoidance (CSMA/CA) protocol for use as a random access protocol technique. Within such a network, when a node receives a packet to be transmitted, it first listens to ensure that no other node is transmitting. If the channel is clear (i.e., no other node is transmitting), the node then transmits the packet. Otherwise, the node chooses a random backoff factor which determines the amount of time the node must wait until it is allowed to retransmit the packet.
The reader should note, however, that collision detection, as is employed in ETHERNET(trademark), cannot be used for the radio transmissions of IEEE 802.11 because transmitting nodes within the wireless LAN cannot hear any other node in the system (network) which may be transmitting. That is, the transmitting node""s own signal is presumably stronger than any other signal arriving at the node. The problem can be analogized to the problem of hearing impairment, that is, some nodes are hearing impaired for any of various reasons.
Under IEEE 802.11, when a mobile wishes to transmit a packet, it may send out a short ready-to-send (RTS) packet containing information on the length of the packet. If the receiving node hears the RTS, it responds with a short clear-to-send (CTS) packet. The transmitting node sends its packet, and, when received successfully by the receiving node, the receiving node sends an acknowledgment (ACK) packet. Problems arise, however, with respect to those receiving nodes which are xe2x80x9chearing impairedxe2x80x9d.
Hidden nodes or stations (STAs) prevent efficient use of bandwidth as a result of their hearing impairment to certain transmissions. For example, FIG. 1 shows an example of a wireless local area network (WLAN) composed of an access point (AP) and a number of stations (STAs). WLAN operation therein is based on the premise that the AP can communicate with all STAs directly over the wireless link while STAs can communicate each other depending on the relative locations due to their limited transmission ranges.
In FIG. 1, STA 1 is seen as clearly able to communicate with STA 2 directly (or in one hop), but not with STA 3. In FIG. 1, a circle around each STA (and the AP) represents the corresponding transmission range, where STAs 1 and 3 are called hidden terminals to each other since they cannot know even the existence of each other without the help of the AP in between. Note that the communication between STAs 1 and 3 should be performed via the AP.
Various attempts have been made to utilize unused portions of TDMA time slots assigned to particular nodes within a WLAN, i.e., to implement channel efficiency. For example, Cheng-Shang Chang, et al., GUARENTEED QUALITY-OF-SERVICE WIRELESS ACCESS TO ATM NETWORKS, IEEE Journal on selected Areas In Communications, vol. 15, no. 1, January, 1997, discloses the use of polling in an effort to realize a protocol for non-preemptive priority for constant bit rate (CBR) and variable bit rate (VBR) traffic which supports ATM services.
U.S. Pat. No. 5,684,791 to Raychaudhuri, et al. (Raychaudhuri) discloses data link procedures for wireless ATM network channel access based on dynamic TDMA (time division multiple access)/TDD(time division duplexing) framework. The system provides ATM services such as ABR data and constant/variable (CBR/VBR) voice using wireless specific MAC and data link control (DLC) protocols. However, a reading of Raychaudhuri shows the complexity of its bi-component MAC protocol and operation.
It is therefore an object of the present invention to provide a wireless MAC protocol, and a wireless LAN system using the MAC protocol which overcomes the shortcomings of conventional methods of efficiently utilizing bandwidth.
It is also an object of the invention to provide a wireless MAC protocol for isochronous traffic support which utilizes the precious wireless link efficiently in a wireless network with hidden terminals.
To that end, the present invention sets forth a medium access control (MAC) protocol for Isochronous traffic support based on an implementation-dependent bandwidth-sharing algorithm. The MAC protocol includes three entities: (1) time slot allocation, which can be considered as the time unit in the underlying wireless network, and will be implementation specific, (2) token passing, which is not token passing as known in the art per se because there is no xe2x80x9crealxe2x80x9d token passed but a xe2x80x9cvirtualxe2x80x9d token is passed, and (3) polling. Since Isochronous traffic requires guaranteed throughput and bounded latency, dynamic time division multiple access (TDMA)-like protocol is used to dynamically reserve bandwidth and allocate time slots to known STAs depending on their needs (demands).
A second embodiment of the invention embodies a wireless local area network (WLAN) which utilizes time slot allocation, virtual token passing and polling where necessary to maximize utilization of bandwidth. The WLAN operates in accordance with the limitations of a medium access control (MAC) protocol for utilization of allowable bandwidth in the WLAN operating at a variable bit rate (VBR) in isochronous transmission mode. The WLAN comprises an access point (AP) and a plurality of mobile stations (STAs), and the MAC protocol includes transmitting a beacon from the AP which includes time slot allocation information for each STA recognized by the AP, and AP information required for new STAs to join the network. The STAs will then transmit during its allocated transmission slot (in the order delegated by the AP). The MAC protocol allows a STA with a xe2x80x9csubsequentlyxe2x80x9d scheduled time slot to utilize any unused time in a time slot allocated to a xe2x80x9cprecedingxe2x80x9d STA when the STA with the subsequently scheduled time slot within the transmission range of the preceding STA. The MAC also generates a polling mechanism at the AP directed to STAs with subsequently scheduled time slots in order to offer the STAs with subsequently scheduled time slots the preceding STA""s remaining time (in its allocated time slot) when the preceding STA has clearly stopped transmitting, and before the time allocated for the subsequent STA to transmit.